Micro display technology is in most cases based on liquid crystal or MEMS technology. Typical projectors based on these technologies are the Liquid Crystal On Silicon (LCOS), Liquid Crystal Display (LCD) and Digital Light Processing (DLP™) projectors. In all these projectors the light source is practically always an Ultra High Performance (UHP) discharge arc lamp. This lamp has a very high power efficiency (lumen per watt) and étendue efficiency (lumen per unit étendue). Because of its small arc length, thus small étendue, it is very attractive for small light valves in micro display projectors.
An important property of any light beam is its étendue. It is a purely geometrical property. In simple wordings, it is the product of the beam cross-section with the spatial angle under which it travels and can be determined by multiplying the diameter or width of the limiting aperture by the angle of divergence of that aperture. The unit of étendue is m2×steradians. From optics theory it follows that the étendue of a beam of light is invariant. If an optical component reduces the étendue of a beam of light, then this automatically implies that some of the light is lost. In a light valve projector, the importance of the étendue is that the optical component with the smallest étendue acceptance acts like a bottleneck for the whole system.
In a well-designed optical system, the étendue acceptances of all optical components are matched with each other. Usually, one of the components determines the étendue of the whole system and the others are matched to this component.
In light valve projectors, the light valve itself tends to be the étendue limiting component. That is a direct consequence of the facts that the price of the light valve component increases very rapidly with its dimensions, and that the light acceptance angle of the light valve is limited if a minimum image contrast is required.
The major consequence of this is that also the étendue of the matching light source is limited, which means that if one wants to increase the light output of the projector, one has to increase the light output of the light source without increasing its size or emission angles.
There are different types of LCOS color projectors, e.g. single panel (color sequential) and three panel systems. There are several known optical architectures for e.g. the three panel system. White light from the UHP lamp is, after some filtering, divided in three basic colors (R, G, B) by a set of dichroic mirrors. Afterwards, each color is independently modulated by another LCOS micro display (light valve). A so-called X-cube combines the three light paths and reflects the image to the projection lens.
This architecture where an UHP lamp is used as light source has many disadvantages. White light has to be divided into its basic components. For this, three dichroic mirrors are needed. This raises the price and enlarges the dimensions of the projector. Presence of UV and IR filters also raises the price. Each component has some losses, so the total optical output will be decreased. After splitting in (R, G, B)-components, each color still has a relatively wide spectrum and this leads to a reduced color gamut.
Another important disadvantage of an UHP lamp is the short lifetime. Normally a component has an average lifetime of 25000 hours. UHP lamps, with 5000 hours, limit the projector lifetime drastically. The other components have a much longer lifetime, therefore it would be advantageous to provide another light engine that could operate for a longer time.
Other disadvantages are the high driving voltage, large volume due to cooling, price, the presence of mercury and danger of explosion. These aspects make the projector also less portable.
There is a great interest in using LEDs (light emitting diodes) or lasers as the light source for a future generation of image projectors. LEDs and lasers have a number of advantages over the now widely used UHP lamps, such as longer lifetime, no explosion hazard, no risk of toxic exhaust at end of life (cf. Hg in UHP lamps), shock resistance, lightweight, very pure basic colours (narrowband) (resulting in a wider colour gamut), possibility to use pulsed addressing (important for colour-sequential operation), and no waste of energy to produce photons with wavelengths that are unwanted (wavelengths between the primary colours).
The major drawback of LEDs to date is the limited light output per unit of emitting area. This is an important limitation for a light source that is considered for use in a light valve based projector.
In recent years, LEDs have become more and more efficient. Many methods to increase the efficiency are based on modifications of the direct LED surroundings (substrate, encapsulation, . . . ) in order to increase the light out-coupling efficiency. These methods as a rule also increase the apparent size of the LEDs, and hence their étendue. Therefore, this does not beneficially influence the efficiency of such a LED in a projection system, unless the étendue of the enlarged LED still matches with the étendue acceptance of the light valve. Nowadays, the maximum LED power that matches with a typical LCOS light valve (0.8″ diagonal) is about 5 Watts.
LEDs are robust and have very long operating time (up to 100 k hours) compared with UHP lamps. This will enlarge the lifetime of the projector drastically. Because of their small size, low operating voltage and the absence of dangerous mercury and explosion hazard, they could be ideal light sources for inexpensive, compact, portable projection systems that even can be run on batteries. The robustness of the light source will result in robust projectors with low maintenance cost.
LEDs are light sources with a narrow spectral emission band, nearly monochromatic, which will lead to a large color gamut. This will increase the number of possible color combinations and thus the image quality. The contrast can be improved by a unique property of LEDs, namely unlike UHP lamps they have a large dimming ratio. By dimming the LEDs for dark images the dynamic range will be adapted and will result in very high contrast. This technique is known as ‘Adaptive Dynamic Range Control’ and is used in classic projectors. The dimming happens there with a mechanical shutter and will be thus easier and faster with LEDs. Another very interesting aspect is the fact that LEDs can be switched rapidly, meaning that LEDs can be pulsed. Pulsing is a very interesting property in case of color sequential architecture, but it can also be used to get a higher light flux in three panel system.
However, there is also an important disadvantage. LED intensities are still too low for actual projection application. The optical power per unit of étendue of a LED is also significantly lower than that of an UHP lamp (approximately 50 times). Even the brightest LEDs cannot equalise the efficiency of this arc lamp. Nowadays this problem is the bottleneck of the LED based projection systems.
An idea that is formulated on many occasions is to combine the light coming from different LEDs in order to increase the light output. The major problem is that most methods to combine the light coming from two separate light sources also add up the étendue of the two light beams. There are a number of known ways to combine two or more light sources without increasing the resulting étendue.
One method is to use a polarising beam splitter (PBS) to combine two light beams with mutually orthogonal linear polarisation, yielding a single beam with mixed polarisation. Unfortunately, this cannot be used for liquid crystal based projectors, because one needs polarised light in order for the light valve to work. Furthermore, LEDs produced today do not generate polarised light, so in order to generate the linearly polarised light, one would first have to split the unpolarised light into two beams with mutually orthogonal polarisation. This would double the étendue of the original light beam, so one ends up with a break-even situation at best. Still, the technique could be used with lasers (which usually do generate polarised light), provided a polarisation-insensitive light valve is used (such as a DLP valve).
A second method is to use an X-Cube or a set of dichroic mirrors and to combine the light coming from light sources with non-overlapping spectra (different colours). For example, a standard X-cube can combine a red, a green and a blue beam into a single white beam with the same étendue. In fact, this technique is already used in all 3-valve projectors. In the case of using UHP lamps, the white light from the lamp is first split into a red, a green and a blue channel, which implies a tripling of the beam étendue, and later it is recombined again to a single beam using an X-Cube. In the case of monochromatic light sources, such as LEDs, each of the light sources is allowed to have the same étendue as the UHP lamp. This is in fact an unlisted advantage of LEDs versus UHP lamps: the light output of the UHP lamp has to be compared with the light output of the red, green and blue LEDs together.
EP 1395064 discloses a projector system, which uses LEDs as the light source. In this document, the LEDs are mounted on a movable support, which is designed to bring the LEDs alternately in front of the optical system. The LEDs are pulsed alternately in order to obtain a higher flux. In an alternative embodiment, a rotatable mirror placed at an angle of 45° is used to reflect the light of alternately pulsed LEDs placed on the inner surface of a cylinder, to guide the light to the optical system of the device. The functioning of this device depends heavily on timing between mechanical elements (positioning devices, mirror rotation) and electronic components (LEDs). Also, mechanical elements are known to fail more easily and to be subject to wear. Further, such a device would be very costly.
To solve the problems mentioned above, another light source is needed.